Coherent waves?

is it necessary ffor coherent waves to be of same frequency? bcoz everywhere i read its written dat they must have a constant phse difference. but cant they have frequencies dat are simple numerical multiples of each other?
like 256 512

There is a constantly changing phase difference between a fundamental and second harmonic. 2πf radians per second, actually. It just happens that, every cycle of the fundamental, the phase is a whole number of cycles.